Modern high-efficiency combustion turbines have firing temperatures that exceed about 2000° F. (1093° C.), and firing temperatures continue to increase as demand for more efficient engines continues. Many components that form the combustor and “hot gas path” turbine sections are directly exposed to aggressive hot combustion gases, for example, the combustor liner, the transition duct between the combustion and turbine sections, and the turbine stationary vanes and rotating blades and surrounding ring segments. In addition to thermal stresses, these and other components are also exposed to mechanical stresses and loads that further wear on the components.
Many of the cobalt-based and nickel-based superalloy materials traditionally used to fabricate the majority of combustion turbine components used in the hot gas path section of the combustion turbine engine are insulated from the hot gas flow by coating the components with a thermal barrier or metallic overlay coating in order to survive long-term operation in this aggressive high-temperature combustion environment.
Thermal barrier coating systems often consist of four layers: the metal substrate, metallic bond coat, thermally grown oxide, and ceramic topcoat. The ceramic topcoat is typically composed of yttria-stabilized zirconia (YSZ), which is desirable for having very low thermal conductivity while remaining stable at nominal operating temperatures typically seen in applications. Such ceramic topcoats can be expensive to apply and/or limited in application methodology.
YSZ is a well known material used to improve the performance of metals used in high temperature applications. The YSZ is applied, typically by a high temperature thermal spray coating process. The YSZ increases the operating temperature of the high temperature substrate metal. In addition, a bond coat is applied between the YSZ and the high temperature metal reducing the thermal expansion mismatch between the YSZ and the high temperature metal, which improves the spallation resistance.
Certain ceramic topcoats are formed as abradable topcoats. Abradable topcoats may be included on components which contact other components, in some cases at high-rotational velocities. Abradable topcoats permit contact between the components while minimizing damage caused by inter-component contact and coating spallation. Topcoats may be adjusted to be abradable by forming the topcoat as a dense vertically cracked layer.
Gas turbine engines can be operated using a number of different fuels. These fuels are combusted in the combustor section of the engine at temperatures at or in excess of 2000° F. (1093° C.), and the gases of combustion are used to rotate the turbine section of the engine, located aft of the combustor section of the engine. Power is generated by the rotating turbine section as energy is extracted from the hot gases of combustion. It is generally economically beneficial to operate the gas turbine engines using the most inexpensive fuel available. One of the more abundant and inexpensive petroleum fuels is crude oil fuel. One of the reasons that crude oil fuel is an economical fuel is that it is not heavily refined. Not being heavily refined, it contains a number of impurities. One of these impurities is vanadium, which forms vanadium oxide (V2O5) at the high temperatures of combustion. Even though MgO is added as a fuel additive and acts as an inhibitor for reaction of vanadium species that forms an inert magnesium vanadate compound on or near the outer surface of the thermal barrier coating, MgO does not completely prevent the attack of YSZ thermal barrier coatings, as vanadium oxide can penetrate microcracks and porosity in the thermal barrier coatings, in particular the vertical cracks of a dense vertically cracked abradable topcoat, providing access not only to the YSZ thermal barrier coating, but also the underlying bond coat. V2O5 is an acidic oxide that can leach yttria from YSZ in cracks and porosity that occur in such thermal barrier coatings. The mechanism of attack is provided by the following reaction:ZrO2(Y2O3)+V2O5→ZrO2(monoclinic)+2YVO4 
Thus, V2O5 maintains the ability to rapidly attack YSZ, causing it to deteriorate and be removed by the hot gas stream. The loss of the TBC exposes the substrate metal and any remaining bond coat to the hot gases of combustion at elevated temperatures. At these elevated temperatures, the substrate metal and the bond coat are subject to corrosion from the hot gases of combustion, which shorten their life. As a result, the components, such as combustors and turbine blades, must be replaced in shorter intervals, which also means additional maintenance time for the turbine during which time it is not producing power. Due to such drawbacks, crude oil fuel has been used in steam boilers having inexpensive components that can be discarded regularly.
The drawbacks of using crude oil fuel can be partially overcome by modifying the composition of the ceramic coating in a gas turbine. For example, gadolinium zirconate can be used instead of YSZ. Although such ceramic coatings resist corrosive attack from calcium-magnesium-aluminosilicate (CMAS), they do not resist corrosive attack from vanadium. In addition, use of gadolinium zirconate alters physical properties in comparison to use of YSZ.
Corrosive attack from vanadium can initially be reduced or eliminated by adding additional layers to a coating system. For example, adding an impermeable barrier layer to YSZ can initially reduce or eliminate corrosive attack from vanadium in HFO. Likewise, laser glazing a top surface of YSZ to form a seal can initially reduce or eliminate corrosive attack from vanadium in crude oil fuel. However, such additional layers can increase cracking tendency upon thermal expansion/contraction associated with operation of a gas turbine, and may inhibit the abradable qualities of an abradable topcoat. This cracking can compromise the additional layer, the YSZ, and/or other layers within the coating system, thereby leading to corrosive attack.